Intel x86 Assembly Fundamentals Computer Organization and Assembly Languages Yung-Yu Chuang 2007/12/10 with slides by Kip Irvine

Intel microprocessor history

Early Intel microprocessors Intel 8080 (1972) –64K addressable RAM –8-bit registers –CP/M operating system –5,6,8,10 MHz –29K transistros Intel 8086/8088 (1978) –IBM-PC used 8088 –1 MB addressable RAM –16-bit registers –16-bit data bus (8-bit for 8088) –separate floating-point unit (8087) –used in low-cost microcontrollers now my first computer

The IBM-AT Intel (1982) –16 MB addressable RAM –Protected memory –several times faster than 8086 –introduced IDE bus architecture –80287 floating point unit –Up to 20MHz –134K transistors

Intel IA-32 Family Intel386 (1985) –4 GB addressable RAM –32-bit registers –paging (virtual memory) –Up to 33MHz Intel486 (1989) –instruction pipelining –Integrated FPU –8K cache Pentium (1993) –Superscalar (two parallel pipelines)

Intel P6 Family Pentium Pro (1995) –advanced optimization techniques in microcode –More pipeline stages –On-board L2 cache Pentium II (1997) –MMX (multimedia) instruction set –Up to 450MHz Pentium III (1999) –SIMD (streaming extensions) instructions (SSE) –Up to 1+GHz Pentium 4 (2000) –NetBurst micro-architecture, tuned for multimedia –3.8+GHz Pentium D (2005, Dual core)

IA-32 Architecture

IA-32 architecture Lots of architecture improvements, pipelining, superscalar, branch prediction, hyperthreading and multi-core. From programmer’s point of view, IA-32 has not changed substantially except the introduction of a set of high-performance instructions

Modes of operation Protected mode –native mode (Windows, Linux), full features, separate memory Real-address mode –native MS-DOS System management mode –power management, system security, diagnostics Virtual-8086 mode hybrid of Protected each program has its own 8086 computer

Addressable memory Protected mode –4 GB –32-bit address Real-address and Virtual-8086 modes –1 MB space –20-bit address

General-purpose registers

Accessing parts of registers Use 8-bit name, 16-bit name, or 32-bit name Applies to EAX, EBX, ECX, and EDX

Index and base registers Some registers have only a 16-bit name for their lower half (no 8-bit aliases). The 16-bit registers are usually used only in real-address mode.

Some specialized register uses (1 of 2) General-Purpose –EAX – accumulator (automatically used by division and multiplication) –ECX – loop counter –ESP – stack pointer (should never be used for arithmetic or data transfer) –ESI, EDI – index registers (used for high-speed memory transfer instructions) –EBP – extended frame pointer (stack)

Some specialized register uses (2 of 2) Segment –CS – code segment –DS – data segment –SS – stack segment –ES, FS, GS - additional segments EIP – instruction pointer EFLAGS –status and control flags –each flag is a single binary bit (set or clear) Some other system registers such as IDTR, GDTR, LDTR etc.

Status flags Carry –unsigned arithmetic out of range Overflow –signed arithmetic out of range Sign –result is negative Zero –result is zero Auxiliary Carry –carry from bit 3 to bit 4 Parity –sum of 1 bits is an even number

Floating-point, MMX, XMM registers Eight 80-bit floating-point data registers –ST(0), ST(1),..., ST(7) –arranged in a stack –used for all floating-point arithmetic Eight 64-bit MMX registers Eight 128-bit XMM registers for single-instruction multiple-data (SIMD) operations

IA-32 Memory Management

Real-address mode 1 MB RAM maximum addressable (20-bit address) Application programs can access any area of memory Single tasking Supported by MS-DOS operating system

Segmented memory Segmented memory addressing: absolute (linear) address is a combination of a 16-bit segment value added to a 16- bit offset linear addresses one segment (64K)

Calculating linear addresses Given a segment address, multiply it by 16 (add a hexadecimal zero), and add it to the offset Example: convert 08F1:0100 to a linear address Adjusted Segment value: 0 8 F 1 0 Add the offset: Linear address: A typical program has three segments: code, data and stack. Segment registers CS, DS and SS are used to store them separately.

Example What linear address corresponds to the segment/offset address 028F:0030? 028F = Always use hexadecimal notation for addresses.

Protected mode (1 of 2) 4 GB addressable RAM (32-bit address) –( to FFFFFFFFh) Each program assigned a memory partition which is protected from other programs Designed for multitasking Supported by Linux & MS-Windows

Protected mode (2 of 2) Segment descriptor tables Program structure –code, data, and stack areas –CS, DS, SS segment descriptors –global descriptor table (GDT) MASM Programs use the Microsoft flat memory model

Flat segmentation model All segments are mapped to the entire 32-bit physical address space, at least two, one for data and one for code global descriptor table (GDT)

Multi-segment model Each program has a local descriptor table (LDT) –holds descriptor for each segment used by the program multiplied by 1000h

Paging Virtual memory uses disk as part of the memory, thus allowing sum of all programs can be larger than physical memory Divides each segment into 4096-byte blocks called pages Page fault (supported directly by the CPU) – issued by CPU when a page must be loaded from disk Virtual memory manager (VMM) – OS utility that manages the loading and unloading of pages

x86 Assembly Language Fundamentals

Instructions Assembled into machine code by assembler Executed at runtime by the CPU Member of the Intel IA-32 instruction set Four parts –Label (optional) –Mnemonic (required) –Operand (usually required) –Comment (optional) Label:MnemonicOperand(s);Comment

Labels Act as place markers –marks the address (offset) of code and data Easier to memorize and more flexible mov ax, [0020] → mov ax, val Follow identifier rules Data label –must be unique –example: myArray BYTE 10 Code label (ends with a colon) –target of jump and loop instructions –example: L1: mov ax, bx... jmp L1

Reserved words and identifiers Reserved words (Appendix D) cannot be used as identifiers –Instruction mnemonics, directives, type attributes, operators, predefined symbols Identifiers –1-247 characters, including digits –case insensitive (by default) –first character must be a letter, or $ –examples: var1Count$first _mainMAXopen_file

Mnemonics and operands Instruction mnemonics –"reminder" –examples: MOV, ADD, SUB, MUL, INC, DEC Operands –constant (immediate value), 96 –constant expression, 2+4 –Register, eax –memory (data label), count Number of operands: 0 to 3 –stc; set Carry flag –inc ax; add 1 to ax –mov count, bx; move BX to count

Directives Commands that are recognized and acted upon by the assembler –Part of assembler ’ s syntax but not part of the Intel instruction set –Used to declare code, data areas, select memory model, declare procedures, etc. –case insensitive Different assemblers have different directives –NASM != MASM, for example Examples:.data.code PROC

Comments Comments are good! –explain the program's purpose –tricky coding techniques –application-specific explanations Single-line comments –begin with semicolon (;) block comments –begin with COMMENT directive and a programmer- chosen character and end with the same programmer-chosen character COMMENT ! This is a comment and this line is also a comment !

Example: adding/subtracting integers TITLE Add and Subtract (AddSub.asm) ; This program adds and subtracts 32-bit integers. INCLUDE Irvine32.inc.code main PROC mov eax,10000h; EAX = 10000h add eax,40000h; EAX = 50000h sub eax,20000h; EAX = 30000h call DumpRegs; display registers exit main ENDP END main directive marking a comment comment copy definitions from Irvine32.inc code segment. 3 segments: code, data, stack beginning of a procedure source destination marks the last line and define the startup procedure defined in Irvine32.inc to end a program

Example output Program output, showing registers and flags: EAX= EBX=7FFDF000 ECX= EDX=FFFFFFFF ESI= EDI= EBP=0012FFF0 ESP=0012FFC4 EIP= EFL= CF=0 SF=0 ZF=0 OF=0

Alternative version of AddSub TITLE Add and Subtract (AddSubAlt.asm) ; This program adds and subtracts 32-bit integers..386.MODEL flat,stdcall.STACK 4096 ExitProcess PROTO, dwExitCode:DWORD DumpRegs PROTO.code main PROC mov eax,10000h; EAX = 10000h add eax,40000h; EAX = 50000h sub eax,20000h; EAX = 30000h call DumpRegs INVOKE ExitProcess,0 main ENDP END main

Program template TITLE Program Template (Template.asm) ; Program Description: ; Author: ; Creation Date: ; Revisions: ; Date: Modified by: INCLUDE Irvine32.inc.data ; (insert variables here).code main PROC ; (insert executable instructions here) exit main ENDP ; (insert additional procedures here) END main

Assemble-link execute cycle The following diagram describes the steps from creating a source program through executing the compiled program. If the source code is modified, Steps 2 through 4 must be repeated.

Defining data

Integer constants [{+|-}] digits [radix] Optional leading + or – sign binary, decimal, hexadecimal, or octal digits Common radix characters: –h – hexadecimal –d – decimal (default) –b – binary –r – encoded real –o – octal Examples: 30d, 6Ah, 42, 42o, 1101b Hexadecimal beginning with letter: 0A5h

Integer expressions Operators and precedence levels: Examples:

Real number constants (encoded reals) Fixed point v.s. floating point Example 3F800000r=+1.0,37.75= r double SEM 1823 SEM ±1.bbbb×2 (E-127)

Real number constants (decimal reals) [sign]integer.[integer][exponent] sign → {+|-} exponent → E[{+|-}]integer Examples: E E5

Character and string constants Enclose character in single or double quotes –'A', "x" –ASCII character = 1 byte Enclose strings in single or double quotes –"ABC" –'xyz' –Each character occupies a single byte Embedded quotes: –‘Say "Goodnight," Gracie’ –"This isn't a test"

Intrinsic data types (1 of 2) BYTE, SBYTE –8-bit unsigned integer; 8-bit signed integer WORD, SWORD –16-bit unsigned & signed integer DWORD, SDWORD –32-bit unsigned & signed integer QWORD –64-bit integer TBYTE –80-bit integer

Intrinsic data types (2 of 2) REAL4 –4-byte IEEE short real REAL8 –8-byte IEEE long real REAL10 –10-byte IEEE extended real

Data definition statement A data definition statement sets aside storage in memory for a variable. May optionally assign a name (label) to the data. Only size matters, other attributes such as signed are just reminders for programmers. Syntax: [name] directive initializer [,initializer]... At least one initializer is required, can be ? All initializers become binary data in memory

Defining BYTE and SBYTE Data value1 BYTE 'A‘ ; character constant value2 BYTE 0 ; smallest unsigned byte value3 BYTE 255 ; largest unsigned byte value4 SBYTE -128 ; smallest signed byte value5 SBYTE +127 ; largest signed byte value6 BYTE ? ; uninitialized byte Each of the following defines a single byte of storage: A variable name is a data label that implies an offset (an address).

Defining multiple bytes list1 BYTE 10,20,30,40 list2 BYTE 10,20,30,40 BYTE 50,60,70,80 BYTE 81,82,83,84 list3 BYTE ?,32,41h, b list4 BYTE 0Ah,20h,‘A’,22h Examples that use multiple initializers:

str1 BYTE "Enter your name",0 str2 BYTE 'Error: halting program',0 str3 BYTE 'A','E','I','O','U' greeting1 BYTE "Welcome to the Encryption Demo program " BYTE "created by Kip Irvine.",0 greeting2 \ BYTE "Welcome to the Encryption Demo program " BYTE "created by Kip Irvine.",0 Defining strings (1 of 2) A string is implemented as an array of characters –For convenience, it is usually enclosed in quotation marks –It usually has a null byte at the end Examples:

Defining strings (2 of 2) End-of-line character sequence: –0Dh = carriage return –0Ah = line feed str1 BYTE "Enter your name: ",0Dh,0Ah BYTE "Enter your address: ",0 newLine BYTE 0Dh,0Ah,0 Idea: Define all strings used by your program in the same area of the data segment.

Using the DUP operator Use DUP to allocate (create space for) an array or string. Counter and argument must be constants or constant expressions var1 BYTE 20 DUP(0); 20 bytes, all zero var2 BYTE 20 DUP(?); 20 bytes, ; uninitialized var3 BYTE 4 DUP("STACK") ; 20 bytes: ;"STACKSTACKSTACKSTACK" var4 BYTE 10,3 DUP(0),20

Defining WORD and SWORD data Define storage for 16-bit integers –or double characters –single value or multiple values word1 WORD ; largest unsigned word2 SWORD –32768; smallest signed word3 WORD ?; uninitialized, ; unsigned word4 WORD "AB"; double characters myList WORD 1,2,3,4,5; array of words array WORD 5 DUP(?); uninitialized array

Defining DWORD and SDWORD data val1 DWORD h ; unsigned val2 SDWORD – ; signed val3 DWORD 20 DUP(?) ; unsigned array val4 SDWORD –3,–2,–1,0,1; signed array Storage definitions for signed and unsigned 32-bit integers:

Defining QWORD, TBYTE, Real Data quad1 QWORD h val1 TBYTE Ah rVal1 REAL rVal2 REAL8 3.2E-260 rVal3 REAL10 4.6E+4096 ShortArray REAL4 20 DUP(0.0) Storage definitions for quadwords, tenbyte values, and real numbers:

Little Endian order All data types larger than a byte store their individual bytes in reverse order. The least significant byte occurs at the first (lowest) memory address. Example: val1 DWORD h

Adding variables to AddSub TITLE Add and Subtract, (AddSub2.asm) INCLUDE Irvine32.inc.data val1 DWORD 10000h val2 DWORD 40000h val3 DWORD 20000h finalVal DWORD ?.code main PROC mov eax,val1; start with 10000h add eax,val2; add 40000h sub eax,val3; subtract 20000h mov finalVal,eax ; store the result (30000h) call DumpRegs; display the registers exit main ENDP END main

Declaring unitialized data Use the.data? directive to declare an unintialized data segment:.data? Within the segment, declare variables with "?" initializers: (will not be assembled into.exe).data smallArray DWORD 10 DUP(0).data? bigArray DWORD 5000 DUP(?) Advantage: the program's EXE file size is reduced.

Mixing code and data.code mov eax, ebx.data temp DWORD ?.code mov temp, eax

Symbolic constants

Equal-sign directive name = expression –expression is a 32-bit integer (expression or constant) –may be redefined –name is called a symbolic constant good programming style to use symbols –Easier to modify –Easier to understand, ESC_key Array DWORD COUNT DUP(0) COUNT=5 mov al, COUNT COUNT=10 mov al, COUNT COUNT = 500. mov al,COUNT

Calculating the size of a byte array current location counter: $ –subtract address of list –difference is the number of bytes list BYTE 10,20,30,40 ListSize = ($ - list) list BYTE 10,20,30,40 ListSize = 4 list BYTE 10,20,30,40 Var2 BYTE 20 DUP(?) ListSize = ($ - list) myString BYTE “This is a long string.” myString_len = ($ - myString)

Calculating the size of a word array current location counter: $ –subtract address of list –difference is the number of bytes –divide by 2 (the size of a word) list WORD 1000h,2000h,3000h,4000h ListSize = ($ - list) / 2 list DWORD 1,2,3,4 ListSize = ($ - list) / 4

EQU directive name EQU expression name EQU symbol name EQU Define a symbol as either an integer or text expression. Can be useful for non-integer constant Cannot be redefined

EQU directive PI EQU pressKey EQU.data prompt BYTE pressKey Matrix1 EQU 10*10 matrix1 EQU.data M1 WORD matrix1; M1 WORD 100 M2 WORD matrix2; M2 WORD 10*10

Addressing

Operand types Three basic types of operands: –Immediate – a constant integer (8, 16, or 32 bits) value is encoded within the instruction –Register – the name of a register register name is converted to a number and encoded within the instruction –Memory – reference to a location in memory memory address is encoded within the instruction, or a register holds the address of a memory location

Instruction operand notation

Direct memory operands A direct memory operand is a named reference to storage in memory The named reference (label) is automatically dereferenced by the assembler.data var1 BYTE 10h,.code mov al,var1; AL = 10h mov al,[var1]; AL = 10h alternate format; I prefer this one.

Direct-offset operands.data arrayB BYTE 10h,20h,30h,40h.code mov al,arrayB+1; AL = 20h mov al,[arrayB+1]; alternative notation mov al,arrayB+3; AL = 40h A constant offset is added to a data label to produce an effective address (EA). The address is dereferenced to get the value inside its memory location. (no range checking)

Direct-offset operands (cont).data arrayW WORD 1000h,2000h,3000h arrayD DWORD 1,2,3,4.code mov ax,[arrayW+2]; AX = 2000h mov ax,[arrayW+4]; AX = 3000h mov eax,[arrayD+4]; EAX = h A constant offset is added to a data label to produce an effective address (EA). The address is dereferenced to get the value inside its memory location. ; will the following assemble and run? mov ax,[arrayW-2]; ?? mov eax,[arrayD+16]; ??

Your turn... Write a program that rearranges the values of three doubleword values in the following array as: 3, 1, 2..data arrayD DWORD 1,2,3 Step 2: Exchange EAX with the third array value and copy the value in EAX to the first array position. Step1: copy the first value into EAX and exchange it with the value in the second position. mov eax,arrayD xchg eax,[arrayD+4] xchg eax,[arrayD+8] mov arrayD,eax

Evaluate this... We want to write a program that adds the following three bytes:.data myBytes BYTE 80h,66h,0A5h What is your evaluation of the following code? mov ax,myBytes add ax,[myBytes+1] add ax,[myBytes+2] What is your evaluation of the following code? mov al,myBytes add al,[myBytes+1] add al,[myBytes+2]

Evaluate this... (cont).data myBytes BYTE 80h,66h,0A5h Yes: Move zero to BX before the MOVZX instruction. How about the following code. Is anything missing? movzx ax,myBytes mov bl,[myBytes+1] add ax,bx mov bl,[myBytes+2] add ax,bx; AX = sum

Data-Related Operators and Directives OFFSET Operator PTR Operator TYPE Operator LENGTHOF Operator SIZEOF Operator LABEL Directive

OFFSET Operator OFFSET returns the distance in bytes, of a label from the beginning of its enclosing segment –Protected mode: 32 bits –Real mode: 16 bits The Protected-mode programs we write only have a single segment (we use the flat memory model).

OFFSET Examples.data bVal BYTE ? wVal WORD ? dVal DWORD ? dVal2 DWORD ?.code mov esi,OFFSET bVal ; ESI = mov esi,OFFSET wVal ; ESI = mov esi,OFFSET dVal ; ESI = mov esi,OFFSET dVal2; ESI = Let's assume that bVal is located at h:

Relating to C/C++ ; C++ version: char array[1000]; char * p = &array; The value returned by OFFSET is a pointer. Compare the following code written for both C++ and assembly language:.data array BYTE 1000 DUP(?).code mov esi,OFFSET array; ESI is p

ALIGN Directive ALIGN bound aligns a variable on a byte, word, doubleword, or paragraph boundary for efficiency. (bound can be 1, 2, 4, or 16.) bValBYTE?; ALIGN 2 wVal WORD ?; bVal2 BYTE ?; ALIGN 4 dValDWORD ?; dVal2 DWORD ?; C

PTR Operator.data myDouble DWORD h.code mov ax,myDouble ; error – why? mov ax,WORD PTR myDouble ; loads 5678h mov WORD PTR myDouble,4321h ; saves 4321h Overrides the default type of a label (variable). Provides the flexibility to access part of a variable. To understand how this works, we need to know about little endian ordering of data in memory.

Little Endian Order Little endian order refers to the way Intel stores integers in memory. Multi-byte integers are stored in reverse order, with the least significant byte stored at the lowest address For example, the doubleword h would be stored as: When integers are loaded from memory into registers, the bytes are automatically re-reversed into their correct positions.

PTR Operator Examples.data myDouble DWORD h mov al,BYTE PTR myDouble; AL = 78h mov al,BYTE PTR [myDouble+1]; AL = 56h mov al,BYTE PTR [myDouble+2]; AL = 34h mov ax,WORD PTR [myDouble]; AX = 5678h mov ax,WORD PTR [myDouble+2]; AX = 1234h

PTR Operator (cont).data myBytes BYTE 12h,34h,56h,78h.code mov ax,WORD PTR [myBytes]; AX = 3412h mov ax,WORD PTR [myBytes+2]; AX = 5634h mov eax,DWORD PTR myBytes; EAX ; = h PTR can also be used to combine elements of a smaller data type and move them into a larger operand. The CPU will automatically reverse the bytes.

Your turn....data varB BYTE 65h,31h,02h,05h varW WORD 6543h,1202h varD DWORD h.code mov ax,WORD PTR [varB+2]; a. mov bl,BYTE PTR varD; b. mov bl,BYTE PTR [varW+2]; c. mov ax,WORD PTR [varD+2]; d. mov eax,DWORD PTR varW; e. Write down the value of each destination operand: 0502h 78h 02h 1234h h

TYPE Operator The TYPE operator returns the size, in bytes, of a single element of a data declaration..data var1 BYTE ? var2 WORD ? var3 DWORD ? var4 QWORD ?.code mov eax,TYPE var1; 1 mov eax,TYPE var2; 2 mov eax,TYPE var3; 4 mov eax,TYPE var4; 8

LENGTHOF Operator.data LENGTHOF byte1 BYTE 10,20,30; 3 array1 WORD 30 DUP(?),0,0; 32 array2 WORD 5 DUP(3 DUP(?)); 15 array3 DWORD 1,2,3,4; 4 digitStr BYTE " ",0; 9.code mov ecx,LENGTHOF array1; 32 The LENGTHOF operator counts the number of elements in a single data declaration.

SIZEOF Operator.data SIZEOF byte1 BYTE 10,20,30; 3 array1 WORD 30 DUP(?),0,0; 64 array2 WORD 5 DUP(3 DUP(?)); 30 array3 DWORD 1,2,3,4; 16 digitStr BYTE " ",0; 9.code mov ecx,SIZEOF array1; 64 The SIZEOF operator returns a value that is equivalent to multiplying LENGTHOF by TYPE.

Spanning Multiple Lines (1 of 2).data array WORD 10,20, 30,40, 50,60.code mov eax,LENGTHOF array; 6 mov ebx,SIZEOF array; 12 A data declaration spans multiple lines if each line (except the last) ends with a comma. The LENGTHOF and SIZEOF operators include all lines belonging to the declaration:

Spanning Multiple Lines (2 of 2).data arrayWORD 10,20 WORD 30,40 WORD 50,60.code mov eax,LENGTHOF array; 2 mov ebx,SIZEOF array; 4 In the following example, array identifies only the first WORD declaration. Compare the values returned by LENGTHOF and SIZEOF here to those in the previous slide:

LABEL Directive Assigns an alternate label name and type to an existing storage location LABEL does not allocate any storage of its own; it is just an alias. Removes the need for the PTR operator.data dwList LABEL DWORD wordList LABEL WORD intList BYTE 00h,10h,00h,20h.code mov eax,dwList; h mov cx,wordList; 1000h mov dl,intList; 00h

Indirect Operands (1 of 2).data val1 BYTE 10h,20h,30h.code mov esi,OFFSET val1 mov al,[esi] ; dereference ESI (AL = 10h) inc esi mov al,[esi] ; AL = 20h inc esi mov al,[esi] ; AL = 30h An indirect operand holds the address of a variable, usually an array or string. It can be dereferenced (just like a pointer). [reg] uses reg as pointer to access memory

Indirect Operands (2 of 2).data myCount WORD 0.code mov esi,OFFSET myCount inc [esi]; error: ambiguous inc WORD PTR [esi]; ok Use PTR when the size of a memory operand is ambiguous. unable to determine the size from the context

Array Sum Example.data arrayW WORD 1000h,2000h,3000h.code mov esi,OFFSET arrayW mov ax,[esi] add esi,2 ; or: add esi,TYPE arrayW add ax,[esi] add esi,2 ; increment ESI by 2 add ax,[esi] ; AX = sum of the array Indirect operands are ideal for traversing an array. Note that the register in brackets must be incremented by a value that matches the array type.

Indexed Operands.data arrayW WORD 1000h,2000h,3000h.code mov esi,0 mov ax,[arrayW + esi] ; AX = 1000h mov ax,arrayW[esi]; alternate format add esi,2 add ax,[arrayW + esi] etc. An indexed operand adds a constant to a register to generate an effective address. There are two notational forms: [label + reg]label[reg]

Index Scaling.data arrayB BYTE 0,1,2,3,4,5 arrayW WORD 0,1,2,3,4,5 arrayD DWORD 0,1,2,3,4,5.code mov esi,4 mov al,arrayB[esi*TYPE arrayB] ; 04 mov bx,arrayW[esi*TYPE arrayW] ; 0004 mov edx,arrayD[esi*TYPE arrayD] ; You can scale an indirect or indexed operand to the offset of an array element. This is done by multiplying the index by the array's TYPE:

Pointers.data arrayW WORD 1000h,2000h,3000h ptrW DWORD arrayW.code mov esi,ptrW mov ax,[esi]; AX = 1000h You can declare a pointer variable that contains the offset of another variable.

Data Transfers Instructions

MOV instruction Move from source to destination. Syntax: MOV destination, source Source and destination have the same size No more than one memory operand permitted CS, EIP, and IP cannot be the destination No immediate to segment moves

MOV instruction.data count BYTE 100 wVal WORD 2.code mov bl,count mov ax,wVal mov count,al mov al,wVal; error mov ax,count; error mov eax,count; error

Your turn... Explain why each of the following MOV statements are invalid:.data bVal BYTE 100 bVal2 BYTE ? wVal WORD 2 dVal DWORD 5.code mov ds,45; a. mov esi,wVal; b. mov eip,dVal; c. mov 25,bVal; d. mov bVal2,bVal; e.

Memory to memory.data var1 WORD ? var2 WORD ?.code mov ax, var1 mov var2, ax

Copy smaller to larger.data count WORD 1.code mov ecx, 0 mov cx, count.data signedVal SWORD -16; FFF0h.code mov ecx, 0 ; mov ecx, 0FFFFFFFFh mov cx, signedVal MOVZX and MOVSX instructions take care of extension for both sign and unsigned integers.

Zero extension mov bl, b movzx ax,bl; zero-extension When you copy a smaller value into a larger destination, the MOVZX instruction fills (extends) the upper half of the destination with zeros. The destination must be a register. movzx r32,r/m8 movzx r32,r/m16 movzx r16,r/m8

Sign extension mov bl, b movsx ax,bl; sign extension The MOVSX instruction fills the upper half of the destination with a copy of the source operand's sign bit. The destination must be a register.

MOVZX MOVSX From a smaller location to a larger one mov bx, 0A69Bh movzx eax, bx; EAX=0000A69Bh movzx edx, bl; EDX= Bh movzx cx, bl; EAX=009Bh mov bx, 0A69Bh movsx eax, bx; EAX=FFFFA69Bh movsx edx, bl; EDX=FFFFFF9Bh movsx cx, bl; EAX=FF9Bh

LAHF/SAHF (load/store status flag from/to AH).data saveflags BYTE ?.code lahf mov saveflags, ah... mov ah, saveflags sahf S,Z,A,P,C flags are copied.

XCHG Instruction XCHG exchanges the values of two operands. At least one operand must be a register. No immediate operands are permitted..data var1 WORD 1000h var2 WORD 2000h.code xchg ax,bx; exchange 16-bit regs xchg ah,al; exchange 8-bit regs xchg var1,bx; exchange mem, reg xchg eax,ebx; exchange 32-bit regs xchg var1,var2; error 2 memory operands

Exchange two memory locations.data var1 WORD 1000h var2 WORD 2000h.code mov ax, val1 xchg ax, val2 mov val1, ax

Arithmetic Instructions

Addition and Subtraction INC and DEC Instructions ADD and SUB Instructions NEG Instruction Implementing Arithmetic Expressions Flags Affected by Arithmetic –Zero –Sign –Carry –Overflow

INC and DEC Instructions Add 1, subtract 1 from destination operand –operand may be register or memory INC destination Logic: destination  destination + 1 DEC destination Logic: destination  destination – 1

INC and DEC Examples.data myWord WORD 1000h myDword DWORD h.code inc myWord ; 1001h dec myWord; 1000h inc myDword; h mov ax,00FFh inc ax; AX = 0100h mov ax,00FFh inc al; AX = 0000h

Your turn... Show the value of the destination operand after each of the following instructions executes:.data myByte BYTE 0FFh, 0.code mov al,myByte; AL = mov ah,[myByte+1]; AH = dec ah; AH = inc al; AL = dec ax; AX = FFh 00h FFh 00h FEFF

ADD and SUB Instructions ADD destination, source Logic: destination  destination + source SUB destination, source Logic: destination  destination – source Same operand rules as for the MOV instruction

ADD and SUB Examples.data var1 DWORD 10000h var2 DWORD 20000h.code; ---EAX--- mov eax,var1; h add eax,var2 ; h add ax,0FFFFh; 0003FFFFh add eax,1; h sub ax,1; 0004FFFFh

NEG (negate) Instruction.data valB BYTE -1 valW WORD code mov al,valB; AL = -1 neg al; AL = +1 neg valW; valW = Reverses the sign of an operand. Operand can be a register or memory operand. Suppose AX contains –32,768 and we apply NEG to it. Will the result be valid?

Implementing Arithmetic Expressions Rval DWORD ? Xval DWORD 26 Yval DWORD 30 Zval DWORD 40.code mov eax,Xval neg eax ; EAX = -26 mov ebx,Yval sub ebx,Zval ; EBX = -10 add eax,ebx mov Rval,eax ; -36 HLL compilers translate mathematical expressions into assembly language. You can do it also. For example: Rval = -Xval + (Yval – Zval)

Your turn... mov ebx,Yval neg ebx add ebx,Zval mov eax,Xval sub ebx mov Rval,eax Translate the following expression into assembly language. Do not permit Xval, Yval, or Zval to be modified: Rval = Xval - (-Yval + Zval) Assume that all values are signed doublewords.

Flags Affected by Arithmetic The ALU has a number of status flags that reflect the outcome of arithmetic (and bitwise) operations –based on the contents of the destination operand Essential flags: –Zero flag – destination equals zero –Sign flag – destination is negative –Carry flag – unsigned value out of range –Overflow flag – signed value out of range The MOV instruction never affects the flags.

Concept Map status flags ALU conditional jumps branching logic arithmetic & bitwise operations part of used by provide attached to affect CPU executes

Zero Flag (ZF) mov cx,1 sub cx,1 ; CX = 0, ZF = 1 mov ax,0FFFFh inc ax ; AX = 0, ZF = 1 inc ax ; AX = 1, ZF = 0 Whenever the destination operand equals Zero, the Zero flag is set. A flag is set when it equals 1. A flag is clear when it equals 0.

Sign Flag (SF) mov cx,0 sub cx,1 ; CX = -1, SF = 1 add cx,2 ; CX = 1, SF = 0 The Sign flag is set when the destination operand is negative. The flag is clear when the destination is positive. The sign flag is a copy of the destination's highest bit: mov al,0 sub al,1 ; AL= b, SF=1 add al,2 ; AL= b, SF=0

Carry Flag (CF) The Carry flag is set when the result of an operation generates an unsigned value that is out of range (too big or too small for the destination operand). mov al,0FFh add al,1; CF = 1, AL = 00 ; Try to go below zero: mov al,0 sub al,1; CF = 1, AL = FF In the second example, we tried to generate a negative value. Unsigned values cannot be negative, so the Carry flag signaled an error condition.

Carry Flag (CF) Addition and CF: copy carry out of MSB to CF Subtraction and CF: copy inverted carry out of MSB to CF INC/DEC do not affect CF Applying NEG to a nonzero operand sets CF

Your turn... mov ax,00FFh add ax,1; AX= SF= ZF= CF= sub ax,1; AX= SF= ZF= CF= add al,1; AL= SF= ZF= CF= mov bh,6Ch add bh,95h; BH= SF= ZF= CF= mov al,2 sub al,3; AL= SF= ZF= CF= For each of the following marked entries, show the values of the destination operand and the Sign, Zero, and Carry flags: 0100h FFh h h FFh 1 0 1

Overflow Flag (OF) The Overflow flag is set when the signed result of an operation is invalid or out of range. ; Example 1 mov al,+127 add al,1; OF = 1, AL = ?? ; Example 2 mov al,7Fh; OF = 1, AL = 80h add al,1 The two examples are identical at the binary level because 7Fh equals To determine the value of the destination operand, it is often easier to calculate in hexadecimal.

A Rule of Thumb When adding two integers, remember that the Overflow flag is only set when... –Two positive operands are added and their sum is negative –Two negative operands are added and their sum is positive What will be the values of OF flag? mov al,80h add al,92h; OF = mov al,-2 add al,+127; OF =

Your turn... mov al,-128 neg al; CF = OF = mov ax,8000h add ax,2; CF = OF = mov ax,0 sub ax,2; CF = OF = mov al,-5 sub al,+125; CF = OF = What will be the values of the Carry and Overflow flags after each operation?

Signed/Unsigned Integers: Hardware Viewpoint All CPU instructions operate exactly the same on signed and unsigned integers The CPU cannot distinguish between signed and unsigned integers YOU, the programmer, are solely responsible for using the correct data type with each instruction

Overflow/Carry Flags: Hardware Viewpoint How the ADD instruction modifies OF and CF: –CF = (carry out of the MSB) –OF = (carry out of the MSB) XOR (carry into the MSB) How the SUB instruction modifies OF and CF: –NEG the source and ADD it to the destination –CF = INVERT (carry out of the MSB) –OF = (carry out of the MSB) XOR (carry into the MSB)

Auxiliary Carry (AC) flag AC indicates a carry or borrow of bit 3 in the destination operand. It is primarily used in binary coded decimal (BCD) arithmetic. mov al, oFh add al, 1; AC = 1

Parity (PF) flag PF is set when LSB of the destination has an even number of 1 bits. mov al, b add al, b; AL= , PF=1 sub al, b; AL= , PF=0

Jump and Loop

JMP and LOOP Instructions Transfer of control or branch instructions –unconditional –conditional JMP Instruction LOOP Instruction LOOP Example Summing an Integer Array Copying a String

JMP Instruction top:. jmp top JMP is an unconditional jump to a label that is usually within the same procedure. Syntax: JMP target Logic: EIP  target Example:

LOOP Instruction The LOOP instruction creates a counting loop Syntax: LOOP target Logic: ECX  ECX – 1 if ECX != 0, jump to target Implementation: The assembler calculates the distance, in bytes, between the current location and the offset of the target label. It is called the relative offset. The relative offset is added to EIP.

LOOP Example The following loop calculates the sum of the integers : When LOOP is assembled, the current location = E. Looking at the LOOP machine code, we see that –5 (FBh) is added to the current location, causing a jump to location :  E + FB B mov ax, B mov ecx, C1 L1:add ax,cx C E2 FB loop L E offset machine code source code

Your turn... If the relative offset is encoded in a single byte, (a) what is the largest possible backward jump? (b) what is the largest possible forward jump? (a)  128 (b) +127 Average sizes of machine instructions are about 3 bytes, so a loop might contain, on average, a maximum of 42 instructions!

Your turn... What will be the final value of AX? mov ax,6 mov ecx,4 L1: inc ax loop L1 How many times will the loop execute? mov ecx,0 X2: inc ax loop X2 10 4,294,967,296

Nested Loop If you need to code a loop within a loop, you must save the outer loop counter's ECX value. In the following example, the outer loop executes 100 times, and the inner loop 20 times..data count DWORD ?.code mov ecx,100; set outer loop count L1: mov count,ecx; save outer loop count mov ecx,20; set inner loop count L2:... loop L2; repeat the inner loop mov ecx,count; restore outer loop count loop L1; repeat the outer loop

Summing an Integer Array.data intarray WORD 100h,200h,300h,400h.code mov edi,OFFSET intarray ; address mov ecx,LENGTHOF intarray ; loop counter mov ax,0 ; zero the sum L1: add ax,[edi] ; add an integer add edi,TYPE intarray ; point to next loop L1; repeat until ECX = 0 The following code calculates the sum of an array of 16-bit integers.

Copying a String.data source BYTE "This is the source string",0 target BYTE SIZEOF source DUP(0),0.code mov esi,0; index register mov ecx,SIZEOF source; loop counter L1: mov al,source[esi]; get char from source mov target[esi],al; store in the target inc esi; move to next char loop L1 ; repeat for entire string good use of SIZEOF The following code copies a string from source to target.

Conditional Processing

Status flags - review The Zero flag is set when the result of an operation equals zero. The Carry flag is set when an instruction generates a result that is too large (or too small) for the destination operand. The Sign flag is set if the destination operand is negative, and it is clear if the destination operand is positive. The Overflow flag is set when an instruction generates an invalid signed result. Less important: –The Parity flag is set when an instruction generates an even number of 1 bits in the low byte of the destination operand. –The Auxiliary Carry flag is set when an operation produces a carry out from bit 3 to bit 4

NOT instruction Performs a bitwise Boolean NOT operation on a single destination operand Syntax: (no flag affected) NOT destination Example: mov al, b not al NOT

AND instruction Performs a bitwise Boolean AND operation between each pair of matching bits in two operands Syntax: (O=0,C=0,SZP) AND destination, source Example: mov al, b and al, b AND bit extraction

OR instruction Performs a bitwise Boolean OR operation between each pair of matching bits in two operands Syntax: (O=0,C=0,SZP) OR destination, source Example: mov dl, b or dl, b OR

XOR instruction Performs a bitwise Boolean exclusive-OR operation between each pair of matching bits in two operands Syntax: (O=0,C=0,SZP) XOR destination, source Example: mov dl, b xor dl, b XOR XOR is a useful way to invert the bits in an operand and data encryption

Applications (1 of 4) mov al,'a‘ ; AL = b and al, b ; AL = b Task: Convert the character in AL to upper case. Solution: Use the AND instruction to clear bit 5.

Applications (2 of 4) mov al,6 ; AL = b or al, b ; AL = b Task: Convert a binary decimal byte into its equivalent ASCII decimal digit. Solution: Use the OR instruction to set bits 4 and 5. The ASCII digit '6' = b

Applications (3 of 4) mov ax,wordVal and ax,1 ; low bit set? jz EvenValue ; jump if Zero flag set Task: Jump to a label if an integer is even. Solution: AND the lowest bit with a 1. If the result is Zero, the number was even.

Applications (4 of 4) or al,al jnz IsNotZero; jump if not zero Task: Jump to a label if the value in AL is not zero. Solution: OR the byte with itself, then use the JNZ (jump if not zero) instruction. ORing any number with itself does not change its value.

TEST instruction Performs a nondestructive AND operation between each pair of matching bits in two operands No operands are modified, but the flags are affected. Example: jump to a label if either bit 0 or bit 1 in AL is set. test al, b jnz ValueFound Example: jump to a label if neither bit 0 nor bit 1 in AL is set. test al, b jz ValueNotFound

CMP instruction (1 of 3) Compares the destination operand to the source operand –Nondestructive subtraction of source from destination (destination operand is not changed) Syntax: (OSZCAP) CMP destination, source Example: destination == source mov al,5 cmp al,5; Zero flag set Example: destination < source mov al,4 cmp al,5; Carry flag set

CMP instruction (2 of 3) Example: destination > source mov al,6 cmp al,5; ZF = 0, CF = 0 (both the Zero and Carry flags are clear) The comparisons shown so far were unsigned.

CMP instruction (3 of 3) Example: destination > source mov al,5 cmp al,-2; Sign flag == Overflow flag The comparisons shown here are performed with signed integers. Example: destination < source mov al,-1 cmp al,5; Sign flag != Overflow flag

Conditions unsignedZFCF destination<source01 destination>source00 destination=source10 signedflags destination<sourceSF != OF destination>sourceSF == OF destination=sourceZF=1

Setting and clearing individual flags and al, 0; set Zero or al, 1; clear Zero or al, 80h; set Sign and al, 7Fh; clear Sign stc; set Carry clc; clear Carry mov al, 7Fh inc al; set Overflow or eax, 0; clear Overflow

Conditional jumps

Conditional structures There are no high-level logic structures such as if-then-else, in the IA-32 instruction set. But, you can use combinations of comparisons and jumps to implement any logic structure. First, an operation such as CMP, AND or SUB is executed to modified the CPU flags. Second, a conditional jump instruction tests the flags and changes the execution flow accordingly. CMP AL, 0 JZ L1 : L1:

J cond instruction A conditional jump instruction branches to a label when specific register or flag conditions are met Jcond destination Four groups: (some are the same) 1.based on specific flag values 2.based on equality between operands 3.based on comparisons of unsigned operands 4.based on comparisons of signed operands

Jumps based on specific flags

Jumps based on equality

Jumps based on unsigned comparisons ＞≧＜≦

Jumps based on signed comparisons

Examples mov Large,bx cmp ax,bx jna Next mov Large,ax Next: Compare unsigned AX to BX, and copy the larger of the two into a variable named Large mov Small,ax cmp bx,ax jnl Next mov Small,bx Next: Compare signed AX to BX, and copy the smaller of the two into a variable named Small

Examples.date intArray DWORD 7,9,3,4,6,1.code... mov ebx, OFFSET intArray mov ecx, LENGTHOF intArray L1: test DWORD PTR [ebx], 1 jz found add ebx, 4 loop L1... Find the first even number in an array of unsigned integers

BT (Bit Test) instruction Copies bit n from an operand into the Carry flag Syntax: BT bitBase, n –bitBase may be r/m16 or r/m32 –n may be r16, r32, or imm8 Example: jump to label L1 if bit 9 is set in the AX register: bt AX,9; CF = bit 9 jc L1; jump if Carry BTC bitBase, n: bit test and complement BTR bitBase, n: bit test and reset (clear) BTS bitBase, n: bit test and set

Conditional loops

LOOPZ and LOOPE Syntax: LOOPE destination LOOPZ destination Logic: –ECX  ECX – 1 –if ECX != 0 and ZF=1, jump to destination The destination label must be between -128 and +127 bytes from the location of the following instruction Useful when scanning an array for the first element that meets some condition.

LOOPNZ and LOOPNE Syntax: LOOPNZ destination LOOPNE destination Logic: –ECX  ECX – 1; –if ECX != 0 and ZF=0, jump to destination

LOOPNZ example.data array SWORD -3,-6,-1,-10,10,30,40,4 sentinel SWORD 0.code mov esi,OFFSET array mov ecx,LENGTHOF array next: test WORD PTR [esi],8000h ; test sign bit pushfd; push flags on stack add esi,TYPE array popfd; pop flags from stack loopnz next; continue loop jnz quit; none found sub esi,TYPE array; ESI points to value quit: The following code finds the first positive value in an array:

Your turn.data array SWORD 50 DUP(?) sentinel SWORD 0FFFFh.code mov esi,OFFSET array mov ecx,LENGTHOF array L1: cmp WORD PTR [esi],0; check for zero quit: Locate the first nonzero value in the array. If none is found, let ESI point to the sentinel value:

Solution.data array SWORD 50 DUP(?) sentinel SWORD 0FFFFh.code mov esi,OFFSET array mov ecx,LENGTHOF array L1:cmp WORD PTR [esi],0; check for zero pushfd; push flags on stack add esi,TYPE array Popfd ; pop flags from stack loope next; continue loop jz quit; none found sub esi,TYPE array; ESI points to value quit:

Conditional structures

Block-structured IF statements Assembly language programmers can easily translate logical statements written in C++/Java into assembly language. For example: mov eax,op1 cmp eax,op2 jne L1 mov X,1 jmp L2 L1: mov X,2 L2: if( op1 == op2 ) X = 1; else X = 2;

Example Implement the following pseudocode in assembly language. All values are unsigned: cmp ebx,ecx ja next mov eax,5 mov edx,6 next: if( ebx <= ecx ) { eax = 5; edx = 6; }

Example Implement the following pseudocode in assembly language. All values are 32-bit signed integers: mov eax,var1 cmp eax,var2 jle L1 mov var3,6 mov var4,7 jmp L2 L1: mov var3,10 L2: if( var1 <= var2 ) var3 = 10; else { var3 = 6; var4 = 7; }

Compound expression with AND When implementing the logical AND operator, consider that HLLs use short-circuit evaluation In the following example, if the first expression is false, the second expression is skipped: if (al > bl) AND (bl > cl) X = 1;

Compound expression with AND cmp al,bl; first expression... ja L1 jmp next L1: cmp bl,cl; second expression... ja L2 jmp next L2:; both are true mov X,1; set X to 1 next: if (al > bl) AND (bl > cl) X = 1; This is one possible implementation...

Compound expression with AND cmp al,bl; first expression... jbe next; quit if false cmp bl,cl; second expression... jbe next; quit if false mov X,1; both are true next: But the following implementation uses 29% less code by reversing the first relational operator. We allow the program to "fall through" to the second expression: if (al > bl) AND (bl > cl) X = 1;

Your turn... Implement the following pseudocode in assembly language. All values are unsigned: cmp ebx,ecx ja next cmp ecx,edx jbe next mov eax,5 mov edx,6 next: if( ebx <= ecx && ecx > edx ) { eax = 5; edx = 6; } (There are multiple correct solutions to this problem.)

Compound Expression with OR In the following example, if the first expression is true, the second expression is skipped: if (al > bl) OR (bl > cl) X = 1;

Compound Expression with OR cmp al,bl ; is AL > BL? ja L1 ; yes cmp bl,cl ; no: is BL > CL? jbe next ; no: skip next statement L1:mov X,1 ; set X to 1 next: if (al > bl) OR (bl > cl) X = 1; We can use "fall-through" logic to keep the code as short as possible:

WHILE Loops while( eax < ebx) eax = eax + 1; A WHILE loop is really an IF statement followed by the body of the loop, followed by an unconditional jump to the top of the loop. Consider the following example: _while: cmp eax,ebx ; check loop condition jae _endwhile ; false? exit loop inc eax ; body of loop jmp _while ; repeat the loop _endwhile:

Your turn... _while: cmp ebx,val1 ; check loop condition ja _endwhile ; false? exit loop add ebx,5 ; body of loop dec val1 jmp while ; repeat the loop _endwhile: while( ebx <= val1) { ebx = ebx + 5; val1 = val1 - 1 } Implement the following loop, using unsigned 32-bit integers:

Example: IF statement nested in a loop while(eax < ebx) { eax++; if (ebx==ecx) X=2; else X=3; } _while: cmp eax, ebx jae _endwhile inc eax cmp ebx, ecx jne _else mov X, 2 jmp _while _else: mov X, 3 jmp _while _endwhile:

Table-driven selection Table-driven selection uses a table lookup to replace a multiway selection structure (switch-case statements in C) Create a table containing lookup values and the offsets of labels or procedures Use a loop to search the table Suited to a large number of comparisons

Table-driven selection.data CaseTable BYTE 'A'; lookup value DWORD Process_A; address of procedure EntrySize = ($ - CaseTable) BYTE 'B' DWORD Process_B BYTE 'C' DWORD Process_C BYTE 'D' DWORD Process_D NumberOfEntries = ($ - CaseTable) / EntrySize Step 1: create a table containing lookup values and procedure offsets:

Table-driven selection mov ebx,OFFSET CaseTable ; point EBX to the table mov ecx,NumberOfEntries ; loop counter L1:cmp al,[ebx] ; match found? jne L2 ; no: continue call NEAR PTR [ebx + 1] ; yes: call the procedure jmp L3 ; and exit the loop L2:add ebx,EntrySize ; point to next entry loop L1 ; repeat until ECX = 0 L3: Step 2: Use a loop to search the table. When a match is found, we call the procedure offset stored in the current table entry: required for procedure pointers

Assignment #4 CRC32 checksum unsigned int crc32(const char* data, size_t length) { // standard polynomial in CRC32 const unsigned int POLY = 0xEDB88320; // standard initial value in CRC32 unsigned int reminder = 0xFFFFFFFF; for(size_t i = 0; i < length; i++){ // must be zero extended reminder ^= (unsigned char)data[i]; for(size_t bit = 0; bit < 8; bit++) if(reminder & 0x01) reminder = (reminder >> 1) ^ POLY; else reminder >>= 1; } return reminder ^ 0xFFFFFFFF; }